charge second battery

Karl Townsend pretended :

Common stuff in recreatioal vehicle world. See the local supplier.

I think this will do the trick.

Its called a battery isolator. They sell them for pickups with campers all the time.

Battery isolators are readily available from normal marine component sources. It is also common to use the 1, 2, 1+2 high current battery selector switches to select the battery or batteries you want to run off of manually.

If you use a diode type isolator, be sure to wire the sense lead from the alternator directly to one of the batteries (doesn't matter which one) *after* the isolator so that the charge voltage is regulated correctly taking into account the diode voltage drop.

But they do NOT function the way the OP envisions.

They charge both together, while allowing the starter to only draw down one battery, and "house" loads to only draw down the other.

Now, if one battery is lower than the other, it WILL take the lion's share of the charge.

That is correct, but make certain this connection is broken when the engine is stopped or all the batteries will discharge through that sense line. It is far better and more reliable to either use a mechanical switch or two separate alternators, one for each system. The voltage drop across isolator diodes can be problematic. Steve

stopped or all the batteries will discharge through

Sense, not field.

separate alternators, one for each

The 1, 2, 1+2 battery switches are very common marine items for a reason.

Karl,

Find a proper marine charger with independent outputs.

There are lots of them available.

The one in my boat is a Xantrex Truecharge 20/40 and has three separate circuits and an optional remote status panel and battery temperature sensor.

boat at sea with that unit. That is a "shore power" charger.(unless there is a major feature not described in the brochure, or I realy missed something)

That's the one on my boat, Clare. Not suggesting he should run out and buy one like it.

Just that marine chargers are better for the marine enviornment. A heck of a lot better than camper stuff.

Karl Townsend wrote in rec.crafts.metalworking on Thu, 16 Dec 2010 05:37:15 -0500:

Isolators suck, use a battery combiner.

But it only woprks when plugged in, right? Runs off of 120vac, not of the engine driven charging system. OK if you have a genset on board.

though.

we got a Pro Mariner: ProSport20 Plus Triple 20 Amps 3 12/24/36 VDC

Does three batteries. We plan one for starter, one user and a spare just sitting there for a failure. Getting somebody to jump you while off shore in a 20 knot wind is a problem.

Karl

I haven't figured this part yet. Unlike any other alternator I've seen there is no sense lead. Just one lead for both voltage sense and current out. I even went to "battery man" repair shop and verified that cummins does it this way. I was wanting to put a key switch to the sense and can't do it. More research needed here.

Karl

If his boat has a problem with that, perhaps an additional auxiliary battery would be in order, and/or a heavier duty charging system. (complete with welding option? AR AR AR!)

Google "charge isolator", which is used in RVs. It won't separate charging times, but they work quite well.

-- Small opportunities are often the beginning of great enterprises. -- Demosthenes

I suppose floating a potato in the salt water is not an option here?

-- Small opportunities are often the beginning of great enterprises. -- Demosthenes

I do.